Tunnelling to extend London Underground's Northern line to Battersea has been completed.
A giant boring machine named Amy broke through at Kennington on Wednesday, meaning a pair of two-mile long tunnels have been created in south London.
The £1.2 billion project is the first major expansion of the Tube since the Jubilee line in the late 1990s.
Two new stations will be opened in 2020 - one at the heart of the Battersea Power Station redevelopment and another at Nine Elms, serving developments such as the US Embassy and the revamped New Covent Garden Market, as well as existing communities.
Mayor of London Sadiq Khan said: "Today's breakthrough is a momentous moment for a project that is going to bring huge benefits to south London.
"The Northern line extension is not only going to make travelling to Battersea and Nine Elms easier, it's also going to bring tens of thousands of new jobs and homes to the area.
"It's another great example of why new infrastructure is so vital to London's success and the wider economy."
Two 650-tonne boring machines operated at depths of 26 metres (85ft) for six months, excavating more than 300,000 tonnes of earth to create the north and southbound tunnels.
They were named in honour of aviation pioneer Amy Johnson, who was the first female pilot to fly solo from Britain to Australia, and Helen Sharman, the first British astronaut.
London's transport commissioner Mike Brown said: "I'm delighted to welcome the two tunnelling machines to Kennington after their epic journey.
"Both machines have had to navigate a challenging, congested path under south London which included sewers, power cables, a Victorian-era well and existing Victoria and Northern line tunnels.
"This achievement brings us a significant step closer to the delivery of the first major Tube extension in nearly two decades."